1. Technical Field
The invention relates to a frequency synthesizer and a frequency synthesizing method. Particularly, the invention relates to a wideband frequency synthesizer capable of improving a frequency operation range by additionally configuring a voltage-controlled oscillator and a frequency synthesizing method thereof.
2. Related Art
In recent years, along with booming development of wireless technology, wireless communication is widely used in people's daily life. Along with quick development of applications of communication techniques, demands on data transmission amount and data transmission speed are greatly increased, and a wide transmission bandwidth is generally required to improve a bit transmission rate. A main challenge of the wireless communication domain lies in a highly integrated and low cost complementary metal oxide semiconductor field effect transistor (CMOSFET) chip, and in a transceiver of a communication system, a frequency synthesizer is generally used to provide a stable local oscillating signal.
In detail, the frequency synthesizer provides the local oscillating signal required by the radio frequency (RF) transceiver in frequency up/down-conversion or linear modulation. Therefore, the quality of the local oscillating signal is an important factor determining a signal transmission quality. Especially, in a multi-carrier system, for example, an orthogonal frequency division modulation (OFDM) communication system, high-density orthogonal carriers have a more strict demanding on frequency spectrum purity of a local oscillation source. In a wireless application of a high application frequency, it must have a high performance requirement on the output signal generated by the frequency synthesizer, so as to generate a clean and stable local oscillating signal for integrating with the wireless transceiver.
Moreover, regarding the conventional frequency synthesizer, since there is only one voltage-controlled oscillator, the frequency operation range thereof is smaller, which cannot satisfy the demand on operation bandwidth in today's multi-standard communication system. Therefore, in order to apply in the system simultaneously complying with a plurality of different communication protocol standards, for example, IEEE 802.11b/g, 802.16d, ultra wide band, etc., the frequency synthesizer is required to have a wide operation frequency range when it is designed. A plurality of wideband frequency synthesizers are successively developed and discussed in recent years. The conventional wideband frequency synthesizer applies a plurality sets of phase-locked loop (PLL) circuits and a plurality sets of frequency dividers in collaboration with a multiplexer, such that the output frequency can fall in different application frequency bands. Moreover, a combination of a plurality of different frequency synthesizers can also result in the fact that the output frequency falls in different application frequency bands, so as to satisfy the demand of the wideband application.
However, in the conventional technique, not only a complicated hardware circuit design is required, a higher circuit cost and power consumption are also required. Regarding the frequency synthesizer having a plurality of frequency dividers, a balance has to be reached between a frequency resolution and the number of devices, and the operation frequency is hard to reach a continuous wideband range. On the other hand, if operation frequencies of two sets of PLL circuits are close to each other, a frequency pulling phenomenon is occurred, which decreases the quality of the output signal. Regarding an integrated circuit design of the frequency synthesizer in the current communication system, not only a high efficiency is required, a low cost is also required, so that it is an important issue to design a better wideband frequency synthesizer satisfying application requirements.